Page:Proceedings of the Royal Society of London Vol 1.djvu/467

Rh of the same mass present also the square arrangement, and accordingly certain portions of it exhibit the octohedral group.

The author remarks, in support of this thcory, that a largc pro- portion of those substances which assume the octahedral form, are considered by chemists as simple bodies, and are therefore more likely to have the simple form of spheres than such as consist of more than one element. Since the supposition of spherical particles appeared to him to aﬁ'ord so satisfactory an explanation of an acknowledged diﬂiculty in crystallography, he was led to consider what other forms would rcsult from the union of solids most nearly allied to spheres; and he observed that obtuse rhomboids, like those of carbonate of lime and other substances, would be formed by the union of oblate spheroids, as indecd Huyghens had long since observed; and that by the union of oblong spheroids, the natural result would be triangular and hexangular prisms, as are found in beryl and phosphate of lime.

But the most singular arrangement noticed, is that which affords an explanation of the origin of cubes in crystallography. These, he supposes, may consist of spherical particles, of two different kinds, regularly intermixed in equal numbers (in conformity to the most re- cent views of binary combination in chemistry); for these, he ob- serves, will not tend, as before, to the octohedral arrangement, but will be perfectly in equilibrio when every group of eight balls com- poses a cube, according to the most obvious course of alternation of the two elements. For in that case all similar balls will be equi- distant from each other, and will also be equally distant from all ad- jacent balls of the opposite denomination.

In a note are subjoined some observations on a theory of crystalli- zation proposed by M. Prechtl, who imagines that a mass of soft spheres may all be compressed into tetrahedra, which is demonstrably impossible. That by another degree of softness or of attraction, spheres, each surrounded by ﬁve others, may be compressed into tri- angular prisms, without regard to the different degree of compression that must take place in the direction of the axis; that other spheres again less compremible than before, and consequently surrounded by as many as six others, may be formed into cubes, which indeed is admitted to be a very possible supposition.

It is observed, however, that M. Prechtl denied that a sphere can be surrounded by more than six, although, in fact, the most probable supposition is, that each soft sphere would be surrounded by twice that number, and would form a mass of regular dodecahedra.

The substance here examined by the author, we are told, was ﬁrst made known by the celebrated Klaproth. It has been ranked as a distinct principle, soluble in water, but insolublc in alcohol or ether, and convertible, by the action of nitric or oxyumriatic acids, into a